This invention relates to head switching control apparatus and, more particularly, to such apparatus which is used in video signal processing apparatus for selectively connecting individual rotary transducer heads, one at a time, to video signal receiving circuitry such that transient noise or pulses which may be produced when the heads are switched, or connected, are timed to occur at substantially the same time during successive video fields.
In a typical video signal recorder, such as a magnetic video signal recorder, two or more rotary magnetic heads scan a recording medium. In one type of recorder, this medium is magnetic tape and the recording system is the so-called video tape recorder (VTR). In a typical two-head VTR, each head scans an oblique track across the magnetic tape. During a recording operation, each track is provided with a video signal field derived from the usual interlaced video fields of, for example, a composite video signal which is about to or has been broadcasted. Thus, each field recorded in a respective track is comprised of video signal information, horizontal synchronizing pulses and the vertical blanking interval. In accordance with a conventional television signal, the vertical blanking interval is formed of a number of equalizing pulses followed by a number of vertical synchronizing pulses followed by another series of equalizing pulses and then a number of horizontal synchronizing pulses preceding the video signal information. In order to record the composite video signal properly in each track, the continuous signals supplied to the VTR must be divided between first one and then the other rotary head. That is, suitable switching apparatus is provided to supply the continuous signal to one head while it scans the magnetic tape, and then to the other head when such other head rotates into contact with the tape. Similarly, during a reproducing, or playback, operation, a switching operation between the two heads must be performed so as to recover a continuous video signal therefrom. That is, when one head scans a track, the previously recorded video signals which are reproduced thereby must be coupled to suitable video signal receiving circuitry; and when the other head rotates into contact with the tape, that head must be switched to the signal receiving circuitry.
One type of head switching control apparatus which has been proposed by the prior art exercises control over the head switching circuitry both during recording and during playback. This proposal recognizes that it is possible to generate a gap from the time one head is disconnected from the signal receiving circuitry until the time that the other head is connected thereto. That is, a gap in the desired continuous video signal may be produced during this head switching, or change-over, period. The prior art suggests that this gap can be avoided if one head reaches the end portion of a track concurrently with the other head reaching the beginning portion of its track, thereby exhibiting some "overlap" in the respective track scans. If this overlap is present during recording, the same information will be recorded by both heads in respective tracks during the overlapping period. Similarly, during signal reproduction, the signals reproduced by one head will be the same as the signals reproduced by the other head during this overlapping period. According to this prior art proposal, if both heads are connected simultaneously to the signal receiving circuitry during this overlapping period, there will be no gap in the continuous video signal which is reproduced. However, at the start of this overlapping period and at the conclusion thereof, switching circuitry is actuated to selectively connect and disconnect the respective transducer heads. Hence, two transient pulses, or noise, will be generated as a function of this head-connect and head-disconnect switching. Although the particular times of occurrences of such transient pulses may be predictable such that clamping or blanking signals can be produced so as to mute or compensate for such noise, there is the possibility that, because of tape shrinkage, small differences in mechanical tolerances among different VTR devices, and the like, the precise times of occurrence of the transient pulses may deviate from the expected times. Consequently, transient noise may be provided during a horizontal line interval in the reproduced continuous video signal, this transient noise appearing as streaks of light in the ultimately reproduced video picture.
In another prior art proposal, head switching control apparatus is used only during a reproducing operation and not during signal recording. Rather, during recording, the composite video signal is applied simultaneously to both rotary heads. Effective switching between these heads is performed automatically by reason of the contact of one or the other of the heads with the magnetic recording medium. That is, the signal supplied to the head which is not in contact with the medium is, of course, not recorded. However, since the same signal is applied to the other head which is in contact with the medium, this signal is recorded in a respective track on the medium. If the heads are spaced apart by 180.degree. and each head scans a track whose effective length is slightly greater than 180.degree., then the end portion of one track will have signals which are the same as those recorded in the beginning portion of the next adjacent track. Typically, the portion of the composite video signal which is recorded in this "overlapping" relation is the vertical blanking interval. During signal playback, a position pulse generator is provided to detect the position of each head relative to the recording medium. For example, a pulse is produced when one head first comes into contact with the medium to scan a track thereacross, and another pulse is produced when the other head first comes into contact with the medium to scan another, adjacent track. These pulses control the head switch-over. That is, when the first head-position pulse is produced, switching circuitry is actuated to couple the corresponding head to the signal receiving circuitry; and when the next head-position pulse is produced, the switching circuitry is actuated to disconnect the first head and to connect the other head to the receiving circuitry.
In the foregoing prior art proposal, a transient pulse, or noise, is produced when the switching circuitry is actuated. Ideally, this transient pulse will be produced at the same time during each vertical blanking interval (i.e., when the playback heads are switched over). However, in practice, because of tape shrinkage, different mechanical tolerances in different VTR systems, slightly different head speeds, and the like, the time of head switch-over, and thus the time that the transient pulse is produced, may vary. That is, the transient pulse may occur at the time of occurrence of an equalizing pulse, or between successive equalizing pulses, or at any other time during the vertical blanking interval. The resultant continuous video signal which is reproduced by the VTR system thus will have a transient pulse which occurs asynchronously at arbitrary locations. Although this may not be noticeable or detrimental in a home entertainment system, this arbitrarily occurring transient pulse is not acceptable for television broadcasting of the reproduced video signal.